The Elements of economic geology

Bibliographic data

Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Chapter

Document type:: Monograph

Structure type:: Chapter

Title:: Part III. Earthy minerals

Collection:: Economics Books

THE MICAS, ASBESTOS, AND GEMS 165 Bort is a black variety of diamond, which grows in radial groups. It is used for cutting brilliants and for the dies in wire drawing. Carbonado is a massive black diamond, which has no cleavage, and is therefore tough as well as hard; it is found in pieces up to 3000 carats in weight ; it was used for the cutting rims of core drills until the rise in price led to its general replacement by cutting bits of iron or steel, and bv chilled shot. Tae CorunpuM GROUP The gems of the corundum group consist of oxide of aluminium (AL,Og). Their value depends on their colour as they lack the brilliance of the diamond. They crystallize in the hexagonal system, their hardness is number 0 on the scale, and they have no cleavage. Corundum crystallizes from a magma that contains an excess of alumina as quartz does from an excess of silica. Corundum is formed mostly with basic calcic rocks because in those rich in alkalis most of the alumina is used as felspar; spinel is formed in those rich in magnesia, and the alumina left is available for cor- undum. Morozewicz (Tsck. Min. and Pet. Mitt., xviii, 1899, Pp. 100-1, 240) showed that a nepheline-basalt when fused with glass dissolves alumina and throws it out during cooling. If the magma cool rapidly, as in dykes, the corundum may be distributed throughout the rock, but in slowly cooling masses it forms on the margin. Hence corundum in commercial quantities forms where an ultra-basic rock is intrusive into rock rich in alumina. Thus the corundum mines of North Carolina occur where dunite (Pratt and Lewis, N. Carol. G.S., i, 1905) intruded gneiss, and the dissolved alumina crystallized on the margin in an irregular sheet or pockets of corundum. Sapphire, the blue variety, has been formed in scattered crystals beside narrow intrusions of basalt in New South Wales (Curran, %.R. Soc. N.S.W., xxx, 1806, p. 235), in Mull (Mem. G.S. Scotl., Mull, 1924, p. 274), and Montana (Pirsson, Amer. ¥. Sci., (4); iv, 1807, p. 42). Most sapphires come from alluvial deposits in Ceylon and are doubtless derived from deep-seated contacts. The most valuable variety is the ruby, which is red and

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